Setup correct scope for initializers and complain about fields initialized more than once.

pkg/front_end/lib/src/fasta/builder/procedure_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.procedure_builder;
 
 // Note: we're deliberately using AsyncMarker and ProcedureKind from kernel
 // outside the kernel-specific builders. This is simpler than creating
 // additional enums.
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
@@ skipping 57 matching lines @@
     if (formals == null) return parent;
     Map<String, Builder> local = <String, Builder>{};
     for (FormalParameterBuilder formal in formals) {
       if (!isConstructor || !formal.hasThis) {
         local[formal.name] = formal;
       }
     }
     return new Scope(local, null, parent, isModifiable: false);
   }
 
+  Scope computeFormalParameterInitializerScope(Scope parent) {
+    // From
+    // [dartLangSpec.tex](../../../../../../docs/language/dartLangSpec.tex) at
+    // revision 94b23d3b125e9d246e07a2b43b61740759a0dace:
+    //
+    // When the formal parameter list of a non-redirecting generative
+    // constructor contains any initializing formals, a new scope is
+    // introduced, the _formal parameter initializer scope_, which is the
+    // current scope of the initializer list of the constructor, and which is
+    // enclosed in the scope where the constructor is declared.  Each
+    // initializing formal in the formal parameter list introduces a final
+    // local variable into the formal parameter initializer scope, but not into
+    // the formal parameter scope; every other formal parameter introduces a
+    // local variable into both the formal parameter scope and the formal
+    // parameter initializer scope.
+
+    if (formals == null) return parent;
+    Map<String, Builder> local = <String, Builder>{};
+    for (FormalParameterBuilder formal in formals) {
+      local[formal.name] = formal.forFormalParameterInitializerScope();
+    }
+    return new Scope(local, null, parent, isModifiable: false);
+  }
+
   /// This scope doesn't correspond to any scope specified in the Dart
   /// Programming Language Specifiction, 4th ed. It's an unspecified extension
   /// to support generic methods.
   Scope computeTypeParameterScope(Scope parent) {
     if (typeVariables == null) return parent;
     Map<String, Builder> local = <String, Builder>{};
     for (TypeVariableBuilder variable in typeVariables) {
       local[variable.name] = variable;
     }
     return new Scope(local, null, parent, isModifiable: false);
   }
+
+  FormalParameterBuilder getFormal(String name) {
+    for (FormalParameterBuilder formal in formals) {
+      if (formal.name == name) return formal;
+    }
+    return null;
+  }
 }